UT4 Sleep Mode is used to extend throttle battery life; it does not charge the throttle batteries but it does reduce the draw on the throttles' batteries.
To put the UT4 to sleep, press and hold any function key down and turn any address selector to the next digit. The throttle will go into sleep mode.
The UT4 will release the address that was selected and the status light will briefly turn red.
If the unit is plugged into LocoNet port you can unplug the throttle and the throttle will then be in sleep mode until plugged back in to LocoNet port.